Another idea the researchers considered is that the jet is heating the dwarf's companion star, causing it to overflow further and dump more hydrogen onto the dwarf. However, the researchers calculated that this heating is not nearly large enough to have this effect.

"We're not the first people who've said that it looks like there's more activity going on around the M87 jet," said co-investigator Michael Shara of the American Museum of Natural History in New York City. "But Hubble has shown this enhanced activity with far more examples and statistical significance than we ever had before."

Shortly after Hubble's launch in 1990, astronomers used its first-generation Faint Object Camera (FOC) to peer into the center of M87 where the monster black hole lurks. They noted that unusual things were happening around the black hole. Almost every time Hubble looked, astronomers saw bluish "transient events" that could be evidence for novae popping off like camera flashes from nearby paparazzi. But the FOC's view was so narrow that Hubble astronomers couldn't look away from the jet to compare with the near-jet region. For over two decades, the results remained mysteriously tantalising.

Compelling evidence for the jet's influence on the stars of the host galaxy was collected over a nine-month interval of Hubble observing with newer, wider-view cameras to count the erupting novae. This was a challenge for the telescope's observing schedule because it required revisiting M87 precisely every five days for another snapshot. Adding up all of the M87 images led to the deepest images of M87 that have ever been taken.

Hubble found 94 novae in the one-third of M87 that its camera can encompass. "The jet was not the only thing that we were looking at – we were looking at the entire inner galaxy. Once you plotted all known novae on top of M87 you didn't need statistics to convince yourself that there is an excess of novae along the jet. This is not rocket science. We made the discovery simply by looking at the images. And while we were really surprised, our statistical analyses of the data confirmed what we clearly saw," said Michael.

"We are witness an intriguing but puzzling phenomenon,” commented Chiara Circosta, an ESA Research Fellow, who studies the impact that accreting supermassive black holes have on the galaxies hosting them in the distant Universe. “I was very surprised by this discovery. Such detailed observations of nearby galaxies are precious to expand our understanding of how galaxies evolve and the broader picture of how jets interact with their host galaxies."

This accomplishment is entirely due to Hubble's unique capabilities. Ground-based telescope images do not have the clarity to see novae deep inside M87. They cannot resolve stars or stellar eruptions close to the galaxy's core because the black hole's surroundings are far too bright. Only Hubble can detect novae against the bright M87 background.

Novae are remarkably common in the Universe. One nova erupts somewhere in M87 every day. But since there are at least 100 billion galaxies throughout the visible Universe, around 1 million novae erupt every second somewhere out there.